Circuit arrangement for automatically closing a switching transistor for a predetermined time period after opening a switching means

Abstract

A light-emitting diode of a photo-coupler is energized and de-energized through a connection to a switching means. Radiation from the light-emitting diode directed toward a photosensitive means of the photocoupler, reduces the resistance of the photosensitive means to rendering a switching transistor conductive. The switching transistor is connected to and controlled by the photosensitive means. When the switching means is open, the switching transistor remains conductive for a predetermined time period due to a transient phenomena of a resistance change of the photosensitive resistor.

What is claimed is: 
     
       1. A circuit arrangement for energizing a load device for a predetermined time period after operation of a switch, comprising: a radiation emissive element, connected in series with said switch between first and second terminals of a voltage source, for emitting radiation in response to a first circuit condition of said switch;   a radiation responsive element having a resistance which varies instantaneously from a normal value in response to receipt of the emitted radiation and gradually returns to the normal value as a function of time in response to the absence of said radiation, said radiation responsive element being connected in series with a resistance element between said first and second voltage source terminals; and   a transistor having a control electrode connected to be responsive to the voltage at a junction between said radiation responsive element and said resistance element, said control electrode being responsive to the voltage at the junction to cause the transistor to assume first and second switching states in response to said voltage being respectively below and above a predetermined threshold level,   whereby there is a transition of a voltage in said circuit arrangement from first to second voltage levels in response to said switch being switched to a second circuit condition and there is a transition of said voltage from said second to first voltage levels in response to said transistor assuming said second switching state.   
     
     
       2. The circuit of claim 1 further comprising a load resistance element connected between said first and second voltage source terminals through said transistor. 
     
     
       3. The circuit of claim 2 wherein said switch is connected in series with an emitter collector path of said transistor. 
     
     
       4. The circuit of claim 3 further comprising a diode connected between said switch and said load resistance element, and a second transistor having a control electrode connected to a junction between said diode and said load resistance element, the load device and said second transistor being connected in series between said first and second voltage source terminals, said diode being poled to provide a conduction path to the control electrode of said second transistor through said switch when the switch is in the first circuit condition. 
     
     
       5. The circuit of claim 4 further comprising a second diode connected between said load resistance element and the control electrode of said second transistor to bias same through the first-mentioned transistor. 
     
     
       6. The circuit of claim 1 wherein said resistance is a variable resistor. 
     
     
       7. The circuit of claim 1 wherein said radiation emissive element is a light-emitting diode and said radiation responsive element is a photodiode having a resistance which is reduced in response to receipt of light from said light-emitting diode and increased as a function of time in response to the absence of said light. 
     
     
       8. In combination, a tape recorder having means for effecting playback of recorded material and means for effecting rewinding of tape, a circuit comprising: a switch, a radiation emissive element connected in series with said switch between first and second terminals of a voltage source, said emissive element emitting radiation in response to a first circuit condition of said switch, a radiation responsive element characterized by a resistance which varies instantaneously from a normal value in response to receipt of said emitted radiation and gradually returns to the normal value as a function of time in response to the absence of said radiation, said radiation responsive element being connected in series with a resistance element between the first and second voltage source terminals, a first transistor having a control electrode connected to be responsive to the voltage at a junction between said radiation responsive element and said resistance element, said control electrode being responsive to the voltage at the junction to cause the transistor to assume first and second switching states in response to said voltage being respectively below and above a predetermined threshold level, whereby there is a transition of a voltage in said circuit from a first to a second level in response to said switch being switched to a second circuit condition and there is a transition of said voltage from said second to first voltage levels in response to said first transistor assuming said second switching state, and a second transistor responsive to the circuit voltage transition and connected to assume a first switching state in response to said voltage transition from the first to the second voltage level and assume a second switching state in response to said voltage transition from the second to first voltage level; said switch being further connected to operate said playback effecting means of said tape recorder, said second transistor being connected to operate said tape rewinding effecting means so that the rewinding means remains activated after the switching means is activated to the second circuit condition. 
     
     
       9. A combination as claimed in claim 8, further comprising a load resistance element connected between the control electrode of said second transistor and one of the controlled electrodes of said first transistor. 
     
     
       10. A combination as claimed in claim 8, wherein said resistance element is a variable resistor. 
     
     
       11. A combination as claimed in claim 9, further comprising a diode connected between said switch and said load resistance element, a junction between said diode and said load resistance element being connected to the control electrode of said second transistor to provide a conduction path to said second transistor control electrode in response to said switching assuming said first circuit condition. 
     
     
       12. A combination as claimed in claim 11, further comprising a second diode connected between a junction of the first-mentioned diode and said load resistance element and said second transistor control electrode. 
     
     
       13. A circuit arrangement for energizing a load device for a predetermined time period after operation of a switch, comprising: a radiation emissive element connected in circuit with said switch between first and second terminals of a voltage source, said element emitting radiation in response to current being supplied to the element by the source in response to the switch being activated to a first circuit condition from a second circuit condition during which current is not supplied to the element;   a radiation responsive element having a resistance which varies instantaneously from a normal value in response to emitted radiation resulting from energization of the switch to the first condition, the resistance of the radiation responsive element gradually returning to the normal value as a function of time in response to the switch being activated to the second condition, said radiation responsive element being connected in series with a resistance element between said first and second voltage source terminals; and   a transistor having a control electrode connected to be responsive to the voltage at a junction between said radiation responsive element and said resistance element, said control electrode being responsive to the voltage at the junction to cause the transistor to assume first and second switching states in response to said voltage being respectively below and above a predetermined threshold level;   whereby there is a transition of a voltage in said circuit arrangement from first to second voltage levels in response to said switch being switched to a second circuit condition and there is a transition of said voltage from said second to first voltage levels in response to said transistor assuming said second switching state.